Detectors for infrared (IR) radiation have many applications. Infrared radiation can often be used to detect objects where visible light is either blocked or not present. Thus, it is possible to use IR detection at night or through clouds, dust, or haze. An IR detector can be in the form of either a single detecting element or an array of such elements to produce an image. A detector that produces such an image is termed a focal plane array.
Infrared radiation can be detected through various ways. One such way is by the use of a material that generates electric charge carriers in response to incident IR radiation. Examples of such photosensitive material are mercury cadmium telluride and a multi-layered structure termed a multiple quantum well (MQW) structure that has alternating layers of wells and barriers.
Depending upon the application, it may be necessary to detect a man-made object in a background of natural objects. It has been found that many natural objects emit IR radiation that has approximately equal magnitudes in both orthogonal polarizations. However, it has been found that most man-made objects tend to emit IR radiation that is significantly biased in favor of one polarization or the other. For this reason, polarization sensitive IR radiation detectors provide one method of distinguishing natural objects from man-made objects.
In still further applications, it may be necessary to accurately determine the temperature of an object. An apparatus that detects the temperature of an object based on measurements at a single radiation wavelength are inaccurate if the emissivity of the object is unknown. A more accurate method for determining the temperature is to take measurements of the object at two different radiation wavelengths. This provides a method of determining the temperature of an object that is less sensitive to the emissivity of the object.